The present invention relates generally to hydrophones and more particularly to hydrophones which employ optical waveguide transducers as sensing elements.
Heretofore, there has been much work related to development of hydrophones for use in sonar towed arrays, fixed coastal or deep water arrays, torpedo guidance systems, and the like. Piezoelectric transducers have been developed for use with these systems, but they have a number of problems. The towed array system has on the order of 250 channels, and each transducer channel consumes power on the order of 25 watts, resulting in approximately 6 kilowatts of power dissipated in the towed array. Furthermore, piezoelectric transducers are low efficiency devices at low acoustic frequencies, on the order of a few hundred hertz or below, and they exhibit non-uniform response at these frequencies. Additionally, these systems are very costly and have reliability problems due to their complexity. Piezoelectric transducers are sealed in a watertight enclosure containing an insulating oil. The transducers become inoperative when wet and cease to function. Thus, the hermetic seal of the enclosure must be excellent for the transducers to perform properly in a water environment.
Because of the above-mentioned difficulties with conventional detection systems, there has been interest in the development of fiber-optic, phase-modulated transducers. However, such transducers are sensitive to environmental factors, such as temperature changes, particularly at low-frequencies. The phase response of the fiber-optic materials to a temperature variation of 1.degree. C. is larger than a response to a pressure change of 1Pa (Pascal) by a factor of more than 10.sup.6. Accordingly, temperature fluctations and the phase shifts resulting therefrom interfere severly with such phase-modulated systems.
Additionally, conventional hydrophone systems employ electrical conductors between the transducer elements and signal processor units. The conductors cause electrical connection problems due to the deterioration of solder joints and loose connector pins. Crosstalk between adjacent channels and elecromagnetic interference also detrimentally affect performance of such conventional systems. Furthermore, many conventional hydrophone systems require the use of preamplifiers located near or at the transducer to provide for sufficient signal strength for transmission to the signal processor. For an additional discussion of the drawbacks of conventional hydrophones, reference is made to U.S. Pat. No. 3,831,137 for "Acousto-Optic Underwater Detector".
Recent developments in the fiber optical waveguide art have led to the development of an optical coupler which utilizes the concept of mode conversion between etched optical fibers. The concept of mode conversion in optical fibers is relatively well-known in the art. Mode conversion in optical fibers as it relates to an optical coupler is discussed in a paper by L. Jeunhomme et al, entitled "Directional Coupler for Multimode Optical Fibers," Applied Physics Letters, October 1976. A system is disclosed therein as it pertains to an experiment for coupling energy out of a multimode optical fiber. The system employs aluminum gratings having opposed rippled surfaces with an optical fiber disposed therebetween. The system and experiments therein relate strictly to the study of mode conversion and optical coupling of energy out of the optical fiber, and does not relate to optical hydrophones for detecting acoustic signals. The system disclosed therein has a fixed lower grating and the upper grating is secured at a fixed position by means of a mechanical screw arrangement. The relative positions between the two gratings are meant to be manually adjustable so as to allow for optimal selective coupling out of the waveguide. Application of this device for the detection of acoustic signals was never considered, and it is neither disclosed nor suggested that the system therein could be used for such an application.
U.S. Pat. No. 4,071,753 entitled "Transducer for Converting Acoustic Energy Directly Into Optical Energy", by Fulenwider et al, discloses a transducer which employs an optical waveguide through which light is transmitted. As shown in FIG. 6 thereof, the light passing through the optical fiber may be modulated by means of bending the waveguide. The optical fiber is placed across two fixed supports and a diaphragm is coupled to the waveguide between the supports by means of a depending member which bares on the optical waveguide. Modulation of the position of the diaphragm in turn causes bending of the optical fiber. The bending of the optical fiber results in modulation of the light transmitted therethrough.